Motor graders are conventionally well-known as a type of construction vehicle for preparing smooth road or ground surfaces. The motor grader is provided with an engine, undercarriage wheels that are rotatably driven by driving force from the engine, and a driving force-transmitting mechanism for transmitting the driving force from the engine to the wheels. Gears can be changed by changing the hydraulic clutch provided to the driving force-transmitting mechanism. The motor grader has a blade that is driven by a hydraulic cylinder. The motor grader moves in a state in which the blade contacts the road surface, whereby a variety of operations such as those given above can be performed (see Japanese Laid-Open Patent Application No. 2003-343325).
In dump trucks, however, a hydraulic-pressure waveform for controlling the clutch is generally modified according to changes in running resistance (see Japanese Laid-Open Patent Application No. 2002-295664). The ratio of speed of the torque converter in the dump truck is used to determine a large, intermediate, or small level of running resistance, and the clutch is controlled using a waveform corresponding to each of the levels. Clutch control can thereby be performed according to the driving conditions, e.g., running on highly uneven construction sites, soft ground, or the like, and shock can be suppressed by changing the clutch.